The IFF acronym is derived from the words Identification, Friend or
Foe. Sometimes it was called Radar Identification and Recognition System.
Starting with the MkI system in WWII, IFF intially started with its own
separate equipment and antennas and had some limited direction finding
capability. In later developments it was incorporated to work with the
main main radar search antenna. Not all radars have an IFF
capability. The radars today that usually have IFF are the Air Search
and Surface Search (long range) using the same interrogator. Today,
the IFF interrogator is embedded in the feedhorn (or a secondary antenna
'bolted' to the parent) of the radar antenna and as the radar sweeps and
hits a target it challenges it at the same time. The operator on
the radar set sees an IFF squawk on his PPI. The latest equipment using
the Mk 12 protocol displays all kinds of information to the challenger.

When IFF is being used, the searching radar automatically sends interrogating
pulses. This is accomplished by having the IFF dipole antenna mounted across
the opening of the feed horn of the radar antenna. The IFF unit in a friendly
target will automatically respond with the correct reply and the reply
is made visible on the screen of the interrogating radar.

This picture shows the placement of the IFF dipole relative to the
main antenna. (Photo courtesy U.S.N).

Having the dipole antenna across the feedhorn was not the case from
the beginning. The next section traces the development of IFF from WWII
to present.

IFF SYSTEM HISTORY

In Britain, IFF was a pre-war development initially destined for installation
on aircraft of the Royal Air Force. Designed by Don Priest and improved
by Bob Carter of the Bawdsey station, the device was housed in an eighteen
square inch box and was carried in the aircraft cockpit behind the pilot.
It was a cumbersome unit for fighter aircraft with little cockpit space.
The unit transmitted a signal to the operator of the Chain Home (CH) radar
to indicate that the aircraft under surveillance was friendly.Failure to
use IFF could cost a pilot his life as evidenced in the Battle of Barking
Creek. This battle occurred in Britain in 1940 when an a CH operator on
the Thames Estuary, not realizing the radar was incorrectly tracking an
aircraft on a 180 degree reciprocal bearing, erroneously identified the
aircraft as an intruder over the North Sea. Fighters were scrambled to
intercept, however they did not turn on their IFF. The investigating fighters
were plotted by a second CH station and reported to Stanmore as intruders.
A second group of fighters was dispatched to intercept the first group.
In the resulting confusion, British fighters fought British fighters and
caused severe damage to a number of machines.

Pilots, who were not familiar with radar, did not appreciate the importance
of switching on the IFF. Alongside the switch to turn on the unit was the
IFF destruct switch to prevent its capture by the enemy. Many a pilot chose
the wrong switch and blew up his IFF unit. The thud of a contained explosion
and the acrid smell of burning insulation in the cockpit did not deter
many pilots from destroying IFF units time and time again. Eventually,
the self destruct switch was secured by a thin wire to prevent its accidental
use. During World War II, both metric and centimetric radars proliferated
on ships of the major navies. Search radars were joined by fire control
sets and ultimately, a ship could locate and destroy air or surface targets
in conditions of zero visibility. It therefore became important to be able
to identify the targets, or at least distinguish friend from enemy.

The earliest American IFF system was the Naval Research Laboratory (NRL)
Model XAE of 1937. It was a shipboard Yagi antenna mounted on a rifle stock
that could be pointed at an unknown aircraft. The pilot would turn on his
omnidirectional identification beacon and the ship would transmit back
an acknowledgement that flashed a light on the aircraft, visible from the
ship. The system worked on a frequency of 500 Mcs. This air to ship system
was tested in 1938 and operational use began in 1939. In England, quite
independently, Watson-Watt conceived a similar device for aircraft identification
that became known as the Mk 1 system. Unfortunately, when aircraft were
in tight quarters at a distance, it was not possible to distinguish friend
from foe. What was needed was a more positive means of identification --
a transponder that would reply to each radar pulse it sensed.

MK I AND MK 2 IFF'S

In 1940, the MK 1 system was introduced in British service. Its details
were disclosed to the United States that fall, but this system was already
obsolete. Quite independently, the Naval Research Labratory (NRL) had developed
a pulse transponder in 1939. To challenge, the radar was switched to a
special pulse repetition frequency (PRF) which triggered responses. In
US Service, the CXAMM IFF system was considered equivalent to the British
Mk II. So far, the radar itself was the interrogator. Since the interrogator
and transponder operated on the radar frequency such operation was not
satisfactory when many different radars were used.

MK III IFF

The Mk III IFF was Watson-Watt's invention and the precursor of modern
IFF systems. IFF challenge and response were to occupy a separate, specialized
band (A-Band; 165 to 185 Mcs or 157 to 187 Mcs, depending on the reference
text). It was adopted as the standard Allied IFF of World War II and remained
in US service for sometime after the war. An important design criterion
was to ensure that the returning signals gave an accurate indication of
target bearing and not merely of target range. The solution was to locate
the IFF interrogator on the radar antenna, rotating with it to give directional
indications. The same device would also receive the response. This was
known as an 'interrogator-responsor' system. Mk III was distributed to
all Allied forces including the Soviet Union. From a post-war point of
view, it could be considered thoroughly compromised.

Because the system relied on active responses from other ships and aircraft,
there were no problems with sea returns. It was possible to use vertical
polarization, thus giving better vertical coverage. The transmitters also
required much less power for a given range so the fitting of IFF presented
few space problems. Initially, the interrogator aerials consisted of directional
Yagi arrays mounted on a horizontal U bar, but in 1944, the most commonly
used aerial system consisted of four, broad-band cage dipoles in a rectangular
configuration and capable of power rotation.

Reports on the value of IFF varied, according to the theatre of operations.
They ranged from "worked very well" to "never saw it used once in two years
at sea."

MK IV IFF

Back in the United States, the NRL designed an alternative system designated
Mk IV. It differed from the Mk III system by employing separate frequencies
(470 and 493.5 Mcs) for challenge and reply. Mk IV was generally held in
reserve during the war in case the Mk III was compromised. A few were used
in the Pacific theatre at the end of the war. In Europe, it was not used
due to its closeness to the frequency of the German Wurzberg radar that
operated at 550 Mcs. A German radar operator might discover IFF pulses
from Allied aircraft, thus compromising the system. Due to its higher operating
frequency (G-band), Mk IV had greater directivity and the typical beam
width was 7 to 10 degrees.

MK V IFF

Mark III was an interim measure and the NRL was directed to produce
a new system that became Mk V/UNB (United Nations Beaconry). Wider transmitter
and receiver frequency separation permitted the use of higher gain antennas
and higher frequencies (950 to 1150 Mcs) made for better directivity. Twelve
channels were made available within this range as an anti-jamming measure.
Signals were coded to permit, for example, identification of one among
several 'friendlies'. On a Plan Position Indicator (PPI) display, transponder
coding would be displayed as a dot and dash elongation (radially) of the
target pip. The first Mk V systems appeared in August of 1944 but the system
did not complete service evaluation until 1947-48. Mk V was considered
successful but few were produced. Installations were confined to CVB's
and fleet carriers where it was important to be able to track and identify
fast targets such as jet aircraft. As an example, Mk V could identify a
jet aircraft flying at 175 mph at 20,000 feet. An attempt was made to re-design
and simplify the Mk V system. This was designated as Mk VI.

MK X IFF

The follow up system to Mk V was Mark X, a system developed in the USA.
At first, this did not mean a jump from the fifth to the tenth Allied IFF
system. The X denoted an experimental system and after it went into production,
it assigned the Mk X (ten) nomenclature. There were problems with the Mk
V system. It used a universal 'code of the day' to distinguish friend from
foe but the NRL considered this a serious, potential security risk. Another
danger in the Mk V was enemy use of Allied IFF to identify our own craft.
By July 1952, the Mk X system started operational use in 50 per cent of
the US Navy. The balance of the fleet was to be converted by January 1954.

The Mk X IFF system sends a pulsed secondary signal from its Interrgator
along with the main radar signal. This in turn is received by a Transponder
situated in the craft under observation. The Transponder then sends back
an appropriate reply that is detected by the Interrogator and distributed
for display. Separate pre-set frequencies are used for interrogation and
reply; 1030 MHz for transmission and 1090 MHz for reply. Normally, the
IFF antenna will rotate in synchronism with the main air warning radar
thus enabling the responses to be superimposed on the radar picture.

Three modes of operation are available for General, Personal and Functional
identification. The mode of operation is determined by the spacing between
the two 1 microsecond pulses which constitute the interrogating signals.
Spacings employed are 3, 5, and 8 microseconds for modes 1, 2 and 3 respectively.
The transponder reply to each of these interrogations is a single one microsecond
pulse except in the case of an aircraft in which the reply to a mode 2
challenge is two, one microsecond pulses spaced 16 microseconds apart.
In addition, the aircraft has the facility of an emergency reply consisting
of four, one microsecond pulses spaced by 16 microseconds between adjacent
pulses. A transponder will always reply to a mode 1 interrogation but replies
to interrogation in modes 2 and 3 are optional. This is dependent upon
the setting of appropriate switches on the transponder control panel. When
set to emergency mode, the aircraft transponder will transmit the four
pulse reply to all modes of interrogation.

MK XII IFF

Mark X did not provide real security. Its interrogation pulse was not
coded. There was always a possibility that an enemy might use Mk X interrogation
pulses to induce US aircraft to identify themselves and then use the aircraft's
IFF system as a homing beacon for missiles. As early as 1951, the NRL had
developed a vacuum tube binary coder but it was too massive for airborne
use. Transistor technology improved matters. By 1956, an American tri-service
group had been formed to implement what has now become the Mk XII system,
the current system in use.

This is a fully cryptographic system so even if the enemy had full knowledge
of the system design, it cannot be used unless the correct interrogation
and response codes are known.

EQUIPMENT TYPES

British IFF systems were coded into two series during World War 2;
the 240 designation was used for interrogators and the 250 series for responders
and beacons. Only IFF equipment types used aboard HMCS Haida are described
here.

242 IFF Interrogator Series

Type 242 IFF series interrogation equipment was fitted on RCN ships
in conjunction with Type 291 and Type 275 radar sets to provide 'A' band
interrogation using the standard Mark III IFF system. Interrogators could
function anywhere in the 165 to 185 MHz band, but were normally used around
179 or 182 MHz (1.8 to 1.6 metres) at a power output of 1 kilowatt. The
pulse repetition frequency was 125 or 50 pulses per second and the pulse
length was 6 microseconds. When used with radar types 291, the 242 was
fitted with aerial outfit 'ASD'. This aerial had an omni-directional radiation
pattern. Type 242 was first introduced into service in 1943. The pulse
repetition frequency of the main radar was counted down in ratios of 4:1
or 10:1 in the modulator which then fired the interrogator transmitter.
Simultaneously, a secondary trace, displaced from the main trace was displayed
as an 'A' scope presentation. Interrogator signals received by the responsor
unit were displayed on this secondary trace as inverted signals along with
the normal radar echoes. Correspondence of the interrogator pulse and the
radar echo identified the target. The associated shipborne transponder
was the type 253 or the Mark III IFF when fitted on an aircraft. Haida
was fitted with the 242WC and 242WS types in the mid 1940's.

242 Type Interrogator

Types: 242 was used with types 271 and 275 radar
242M was
used with radar types 276/277 and 293.
242P/Q
were used with radar sets 960/982/983.

The pulse repetition and pulse length varies depending on the types
of radar that the 242 was used with. 242M is similar to that of type 242
except for some minor modifications. Transmitter power was now selectable
between low and high power outputs (2 kw or 10 kw) and the output frequency
could be varied across a 30 MHz wide band. A pre-amplifier was incorporated
in the transmitter unit that increased the responsor range and the transmit/receive
(T/R) switching arrangements were improved. When the 242M operated in conjunction
with the 276/277/293 radar types, it was fitted with aerial outfit 'ASS'.
This aerial consisted of four broadband vertical dipoles with power rotation.

242Q. The power output of the Q
variant was 2 or 10 kw. (Photo courtesy of the British Admiralty)

242 antenna. This was used with type 271 radar aboard HMCS HAIDA in
1944. (Extract from CB 4182)

Close up view of 242M Aerial Outfit ASS.
(Photo courtesy of the British Admiralty)

Aerial Outfit 242M on HMCS HAIDA as see on her
first tour of duty in Korea. (Photo courtesy RCN)

253P Transponder

Type 253P was a shipborne transponder, compatible with the Mark III
IFF system and operated in response to triggering pulses from any interrogator
or radar set in the same frequency band. When triggered, Types 253P/Q responded
with various coded signals as required for the purposes of normal interrogation
or ship-to-ship identification or homing. This set could operate in the
157 to 187 MHz band but normally operated at 182 MHz for ship-to-ship identification
or when used as a beacon facility. In a normal fit, aerial outfit 'ASH'
was used. For installation aboard coastal craft, aerial outfit 'ANT' was
used. Normally the power output was 10 watts. A low power setting of 0.75
watts was available as an anti-direction finding measure. The pulse repetition
frequency was triggered by the receipt of signals from interrogators of
radar sets. This was limited only by a 300 microsecond period of quiescence
between each transmission. The pulse length of the output signal could
be set for narrow (6-10 us), wide (17-25 us), or distress mode (80 us).

Types 253P and 253Q were similar except that type 253Q was
mounted in a resilient steel cabinet. By operating the buttons marked 'I',
'A', and 'B' on the code selection unit, the following operating conditions
were permitted:

I: Normal Mark III IFF responses (sweeping 157-187 MHz every
2.8 seconds). Six codes were available by selection and each code consisted
of four transmissions using narrow and wide pulses. A complete code was
transmitted once every 11.2 seconds. A special extra wide pulse was available
for distress purposes.

A: Alternate normal IFF codes consisting of four narrow pulses
for 5.6 seconds followed by 5.6 seconds of Identity Code on a fixed frequency
of 182 Mcs. This code consisted of two letters that could be of any combination
of nine narrow, wide or blank pulses which were selected by means of the
nine switches on the Code Selection Unit.

C: Chopped response on the frequency of 182 MHz. The response
was mechanically interrupted for 40 milliseconds every one-fifth second
to distinguish it from a normal code.

To limit mutual interference, the antenna for type 253P was situated at
least 12 feet or more from the nearest interrogator antenna on a ship.
Haida was fitted with the type 253P transponder during the mid 1940's.

Modern day IFF systems are basically Question/Answer systems.
An interrogator system sends out a coded radio signal that asks any number
of queries, including: Who are you? The interrogator system is frequently
associated with a primary radar installation, but it may also be installed
aboard a ship or another airplane. The interrogation code or challenge,
as it is called, is received by an electronic system known as a transponder
that is aboard the target aircraft. If the transponder receives the proper
electronic code from an interrogator, it automatically transmits the requested
identification back to the interrogating radar. Because it was developed
as an adjunct to the primary echo-type detection radar and is usually used
in conjunction with a primary radar, the IFF system is also known as secondary
radar.

Modern IFF is a two channel system, with one frequency (1030 MHz) used
for the interrogating signals and another (1090 MHz) for the reply. The
system is further broken down into four modes of operation, two for both
military and civilian aircraft and two strictly for military use.

Each mode of operation elicits a specific type of information from the
aircraft that is being challenged.

Mode 1, which has 64 reply codes, is used in military
air traffic control to determine what type of aircraft is answering or
what type of mission it is on.

Mode 2, also only for military use, requests the "tail
number" that identifies a particular aircraft. There are 4096 possible
reply codes in this mode.

Mode 3/A is the standard air traffic control mode. It
is used internationally, in conjunction with the automatic altitude reporting
mode (Mode C), to provide positive control of all aircraft flying under
instrument flight rules. Such aircraft are assigned unique mode3/A codes
by the airport departure controller. General aviation aircraft flying under
visual flight rules are not under constant positive control, and such aircraft
use a common Mode 3/A code of 1200. In either case, the assigned code number
is manually entered into the transponder control unit by the pilot or a
crew member.

Altitude information is provided to the transponder by the aircraft's
air data computer in increments of 100 feet. When interrogated in Mode
C, the transponder automatically replies with the aircraft altitude. Ground
interrogators normally interlace modes by alternately sending Mode 3/A
and Mode C challenges thus receiving continuous identity and altitude data
from the controlled aircraft.

A timing diagram which shows the timing of the pulses for various IFF
modes. (Graphic courtesy Litton Systems)

The code signal sent by the interrogator system consists of two pulses
spaced at a precisely defined interval. (A third pulse that has nothing
to do with the coding of the query is actually used for interference suppression
reasons.) In Mode 1, the interval between the first and last pulse is 3
microseconds; in Mode 2, it is five microseconds; in Mode 3/A, it is eight
microseconds; and, in Mode C, it is 21 microseconds. The airborne transponder
contains circuitry that discriminates between these various timings and
automatically sends back the desired reply.

The transponder replies are also in the form of a pulse, though in this
case, there are 12 information pulses that are digitally coded as "ones"
and "zeros." The total number of reply code combinations therefore, is
4,096. The reply codes are entered by means of four code wheels on the
transponder control unit. The reply pulses generated by the transponder
are decoded by the interrogating system and are typically displayed as
needed on the primary radar scope near the blip that represents the aircraft
that has been challenged. Thus, the aircraft controller can monitor the
track of each aircraft through his zone and know its identity, altitude
and position at all times.

The original reason for IFF systems came about was to identify friendly
forces in a battlefield environment. For that reason, it is essential that
hostile forces not be able to use the system to identify themselves as
friendly even if the physical IFF equipment should fall into their hands.

The secure mode is used exclusively for military purposes.
This mode uses a very long challenge word which contains a preamble that
tells the transponder it is about to receive a secure message. The challenge
itself is encrypted at the interrogator by a separate device that uses
various mathematical algorithms to put it in a secure form. The transponder
routes the ensuing challenge to a separate device that uses the inverse
algorithms to decode the challenge. In effect, each challenge is telling
the transponder to respond in a certain way. If the transponder cannot
decipher the challenge, it will not be able to respond in the proper way
and thus will not be identified as a friend.

To prevent unauthorized use of either the interrogation equipment or
the transponders if they should fall into hostile hands, a key code must
be periodically entered into each device. To eliminate the chance of a
random guess by a hostile target corresponding with the proper response,
each identification consists of a rapid series of challenges each requiring
a different response that must be correct before the target is confirmed
as a friend. A very high degree of security to the identification system
is ensured through the use of key codes and powerful cryptographic techniques.

[Acknowledgments to Litton Systems the information on modern day
IFF].

When HMCS HAIDA paid off, this was her IFF fitting. As information
becomes available, more details will be provided. Unless otherwise noted,
all the equipment was fitted in the EMR compartment.

F. Oorschot, from the Netherlands provides a UPX-1 description."The
UPX series were used on the destroyers and frigates of the Dutch navy
in the years 1950 to 1970.

UPX-1 was the IFF (Mk X) interrogator; Tx Freq. 1090 Mc/s, Rx Freq.1030
Mc/s. Average power - 1 kw. using Modes 1, 2 and 3. Mode 1
and 2 was a naval-airforce mode. Mode 3 was also a airborne civilian mode.
The time between the two send pulses depended on the mode. Mode 1 was 3
usec. Mode 2, was 5 usec while Mode was 3.8 usec. With the code-selectors,
you could select a video code that was filtered for a display on
a PPI screen.

With the big switch, in the middle and under the UPX-24, you could select
the kind of video for the display.
The IFF antenna was fitted on the feeder of a long range early air
warning radar (L-band). Trigger for the transmitter came from same radar.
The UPX-1 transmitting mode was also controlled by the AN/UPX-24".

The KY-200/UPX-12 was a unit of transponder set AN/UPX-12. Not fitted
on HAIDA but used by the RCN. Very similar to the UPX-1 except for the
handles and the left side of the panel below the meter. (Photo
by Richard Brisson - used with permission)

C1008/UPA-24

This was the control head for the group video decoder. It controlled the
mode of transmission ( ie mode 1, 2 or 3) and the 'squawk' code assigned
to each particular ship. (Photo by Jerry Proc)

From bottom to top: PP-2391 Power Supply; SM-189/UPM99 Code Simulator
; TS-1253-UP Radar Test Set; CY-1156/UPM-4 Accessory Box. The TS-1253 unit
can function separately since it has an internal power supply and incorporates
the functions of a precision oscilloscope and SlF code generator.

Radar Test Set AN/UPM-99 is designed for testing and performing corrective
maintenance on IFF equipment of both the Mark X and SIF (Selective Identification
Feature) type. It can also be used for making various tests required for
maintenance of other radar equipment operating within the 925 to 1225 MHz
frequency range.

The entire test set incorporates 88 vacuum tubes and 72 solid state
diodes and was introduced in 1960. it was intended for HMCS HAIDA but was
never fitted.